The present invention relates to a tantalum or niobium powder that is useful as a material for a positive electrode to be incorporated into a solid electrolytic capacitor and to a process for producing the powder, and more particularly to a tantalum powder that can provide a positive electrode endowed with low equivalent series resistance (abbreviated as xe2x80x9cESRxe2x80x9d hereinafter) and high capacitance.
Conventionally, a positive electrode in a solid electrolytic capacitor is formed of a porous sintered tantalum or niobium body having a porosity of, for example, 70% by volume. Tantalum or niobium powder serving as a raw material for producing the porous sintered tantalum or niobium body is an agglomeration of particles assuming the form of a sponge containing a large number of quasi-spherical pores that communicate with one another. The agglomerated particles have a spherical shape with a particle diameter of several tens to several hundred xcexcm.
Conventionally, tantalum powder is produced in the following manner, for example.
First, a primary powder is prepared by way of a known method such as reduction of potassium heptafluorotantalate by sodium or reduction of tantalum pentachloride by hydrogen. Then, the thus-obtained primary powder is subjected to washing with acid/water as needed, degassing, and heat treatment at a temperature at least 1000xc2x0 C., followed by deoxidation treatment to remove excess oxygen, to thereby obtain a tantalum powder.
Subsequently, the thus-obtained tantalum powder is subjected to press working into a predetermined shape and then to sintering, to thereby obtain a positive electrode having a large number of pores derived from the pores contained in the agglomerated particles (i.e., tantalum powder).
Nobium primary powder is produced by the same process after the reduction of potassium heptafluoroniobate by sodium or reduction of niobium pentachloride by hydrogen. And a positive electrode can be obtained from press working and sintering the powder.
A solid electrolytic capacitor may be produced in the following manner, for example. On the surface of the porous sintered body (the positive electrode), a film of a solid electrolyte (hereinafter called a solid electrolyte film) is formed. Onto the film, a negative electrode formed of an Ni wire or a like material is bonded by soldering. The porous sintered body and the negative electrode are then covered integrally with a coating resin such as flame-resistant epoxy resin.
Conventionally, manganese oxide has predominantly been used as a solid electrolyte. A solid electrolyte film formed of manganese oxide may be produced in the following manner, for example. First, the porous sintered body is subjected to chemical forming through a customary method. Then, the porous sintered body is soaked in a solution of manganese nitrate and pyrolyzed, to thereby form a solid electrolyte film. Since the porous sintered body contains a large number of quasi-spherical pores communicating with one another as described above, when the porous sintered body is soaked in the solution of manganese nitrate, the solution permeates through the pores on the surface of the sintered body, reaching the pores inside the sintered body that communicate with the pores on the surface, and then to the pores on another portion of the surface of the sintered body. In this manner, the solution of manganese nitrate permeates the entirety of the porous sintered body. Accordingly, a solid electrolyte film having a large area is formed, enabling efficient use of the entire surface of the positive electrode.
Recently, as electronic apparatus and circuits have been down-sized and have high frequency, there is an increasing demand for solid electrolytic capacitors endowed with high capacitance and low ESR. The capacitance of a solid electrolytic capacitor increases with the surface area of the positive electrode present therein. Thus, the porous sintered tantalum body preferably has high porosity, to thereby produce a solid electrolytic capacitor endowed with high capacitance.
Also, when the positive electrode present in a solid electrolytic capacitor assembled into a CPU or power circuit of a personal computer has an increased ESR, a failure may occur in signal processing with high-speed operation in the electronic circuits. Thus, ESR is an important characteristic.
The primary cause for an increase in ESR of the positive electrode is a failure to form a solid electrolyte film.
However, an increase in porosity of tantalum or niobium powder in order to produce a porous sintered body of higher porosity may deteriorate the strength of the tantalum or niobium powder, causing crushing of pores during press working and resulting in decreased porosity of the porous sintered body. Thus, the porosity of tantalum or niobium powder must be adjusted to a level that endows the tantalum or niobium powder with appropriate strength.
Also, a failure to form a solid electrolyte film is predominantly caused by the heterogeneity of the pores contained in the porous sintered body.
Accordingly, when the pores inside the porous sintered body do not communicate with the pores on the surface of the porous sintered body; i.e., when the pores inside the sintered body are closed and independent, the solution of manganese nitrate does not permeate these closed pores, resulting in a failure to form a solid electrolyte film. Also, when a pore inside the sintered body communicates with one pore on the surface of the sintered body but not to any other pore on another part of the surface of the sintered body, pot-shaped pores having a bottom are formed. The solution of manganese nitrate does not sufficiently permeate such a pore, resulting in a failure to form a solid electrolyte film. FIG. 7 is a microphotograph obtained by use of a scanning electron microscope (SEM) showing a porous sintered tantalum body produced from conventional tantalum powder and containing closed pores and pot-shaped pores.
Generation of the closed pores and pot-shaped pores depends on factors such as the particle size distribution of the powder, the crushing resistance of pores present in the powder during press working (ease of compaction), and the state and fraction of pores inside the powder.
Recently, a new technique that utilizes conductive polymers instead of manganese oxide has been put into practical use. Since conductive polymers are of large molecular size, they encounter difficulty in permeating the pores present in the porous sintered body. Thus, more precise control over the pores present in the porous sintered body is required.
The present invention has been attained in view of the foregoing, and an object of the present invention is to obtain a tantalum or niobium powder that can provide a porous sintered body having a large surface area and a lower risk of failure in forming a solid electrolyte film.
More specifically, an object of the present invention is to provide a tantalum or niobium powder having homogeneous pores and endowed with appropriate strength.
Still more specifically, an object of the present invention is to provide a technique that can reduce the risk of producing closed pores and pot-shaped pores.
The present inventors have found that a tantalum or niobium powder in the form of columnar particles can be obtained through a specific production process, thus leading to completion of the present invention.
Accordingly, in order to solve the above-mentioned drawbacks, the present invention provides the following.
A first embodiment is directed toward providing a tantalum or niobium powder comprising aggregated columnar particles.
A second embodiment is directed toward providing a tantalum powder according to the first embodiment, wherein the powder contains radially aggregated particles formed by aggregating a plurality of columnar particles radially.
A third embodiment is directed toward providing a tantalum powder according to the first or second embodiment, wherein the powder is obtained by reduction of a tantalate salt without allowing the tantalate salt to come into direct contact with a reducing agent. And the embodiment is also directed toward providing a niobium powder according to the first embodiment, wherein the powder is obtained by reduction of a niobate salt without allowing the niobate salt to come into direct contact with a reducing agent.
A fourth embodiment is directed toward providing a tantalum or niobium powder according to the third embodiment, wherein the reducing agent comprises magnesium or sodium.
A fifth embodiment is directed toward providing a process for producing a tantalum or niobium powder, wherein the tantalum or niobium powder is obtained by reduction of a tantalate or niobate salt without allowing the tantalate or niobate salt to come into direct contact with a reducing agent.
A sixth embodiment is directed toward providing a process for producing a tantalum or niobium powder according to the fifth embodiment, wherein the reducing agent comprises magnesium or sodium.
A seventh embodiment is directed toward providing a porous sintered tantalum or niobium body formed of a tantalum or niobium powder as recited in any one of the first through fourth embodiments.
An eighth embodiment is directed toward providing a solid electrolytic capacitor, which contains a positive electrode formed of a porous sintered tantalum or niobium body as recited in the seventh invention.